Radar reflector



May 22, 1956 M. BRUCKER RADAR REFLECTOR Filed June 20, 1952 F 6 V W LN L6 an Wm WM 1 c Mm F H B A Pww a. se 0 NV N 5 M W W 2 w a 5 wm M w E UKEF. c M? BF mm m a r N LE UM ww M A a w 0 h w P R m m mw 6 A m MIL TONBRUCKER, INVEN TOR.

HiM 6 EEN United States Patent RADAR REFLECTOR Milton Brucker, LosAngeles, Calif., assignor to Zenith Plastics Company, Gardena, Calif., acorporation of California Application June 20, 1952, Serial No. 294,632

7 Claims. (Cl. 2343-18) This invention relates to means and method forconstructing a radar reflector. More particularly it relates to a radarreflector construction of new and improved light weight and rigiditymade of a minimum of metal while obtaining all the advantages normallyattributed to all-metal reflectors. It likewise relates to an improvedmethod of making the desired reflector, largely of plastic and fibrousmaterials.

Heretofore commercially acceptable radar reflectors, for use inparticular by the Armed Forces of the United States for aircraft,seagoing, and land use, depended upon use of an all-metal concavity, orthe like, all-metal shield, either cast integrally as a reflective unit,or made reticulate by metal bars, rods, strip, mesh and the like. Suchprior art devices are relatively heavy in weight, costly in theirmanufacture and maintenance, and include the use of metal predominantlyif not ex clusively. lnasmuch as the larger radar reflectors are mosteffective in use, the weight-size relationship becomes critical,particularly for aircraft use. It is also important, to a somewhatlesser extent however, on ocean-going vessels, and even on land.

In view of the above considerations and others, it is among the objectsof this invention to provide from readily available plastic and fibrousmaterials, a satisfactory and commercially acceptable radar reflectorutilizing a minimum of metal but Without thereby diminishing either theutility or durability of a radar reflector of comparable size made inaccordance with prior art teachings.

It is another object of this invention to provide, in a radar reflectorof the desired character described, a weathermesistant, non-metallicsurface, a relatively fine layer of reflective metal, and a relativelystiff and substantial highly reticulate backing of utmost lightnessconsistent with strength and durability, and in a manner correspondingto the first mentioned non-metallic surface which is closely adherent toand provides a strong backing and protection to the reflective metallayer.

Yet another object of the invention is the provision of a new andimproved plastic construction for radar reflectors incorporating metalas the electronically effective ingredient, and the plastic as adurable, weather-resistant and non-corrosive structural foundation.

Yet another object of the invention is to provide a plastic radarreflector construction capable of satisfying the rigid standards of theArmed Forces of the United States. as well as commercial requirements,and whose utility is substantially unaffected by increases in size, saidincrease being accompanied by disproportionate increases in weight,foundation and reinforcement.

it is among the objects of this invention to provide a new and improvedmethod of making a radar reflector as herein described and claimed.

Yet another object of the invention is the provision of a new andimproved method for insuring sufficient rigidity of the metal reflectorportion of the device of the instant invention.

2,747,180 Patented May 22, 1956 It is moreover an object to provide animproved method for insuring permanent adherence of the several metaland non-metal parts of the desired structure to one another.

It is a further object of the invention to provide new and improvedmeans and molding method for obtaining a radair reflector of the instantinvention.

in addition, it is among the objects of this specification and inventionto set forth a suggested, preferred means and method for obtaining a newand improved radar reflector construction in accordance with theinventors conception; to improve prior art devices and methodsheretofore intended to accomplish generally similar purposes.

These and other objects and purposes will be more fully understood byreference to the accompanying specification considered in the light ofthe drawings and the appended claims.

In the drawings:

Figure l is a perspective view, partially in section, of a mold andradar reflector constructed thereon in accordance with the teaching ofthis invention.

Figure 2 is a flow diagram of the method of this invention.

Figure 3 is a fragmentary sectional view of one edge of a finishedreflector embodying this invention.

Referring more particularly to the drawings, there is illustrated by wayof example but not of limitation a mold 10 of plastic or the like,having a base 11 by which the same is supported in any suitable fashion,as upon a floor, and provided with any reinforcing rim, as of wood orsteel. The upper surface 13 of the mold is convex to conform to thedesired reflective shape of the intended reflector surface. The mold isfurther preferably provided with an annular flange portion 14 for apurpose to be described.

In the construction of the instant reflector, a layer of fiberglasscloth 15, thoroughly impregnated with a resin, is closely laid over theupper surface of. the convex portion 13 of the mold, and also to theupper surface 16 of the flange 14 thereof. The resin is preferably alsoapplied, prior to the application of the impregnated fiberglass sheet15, to the mold surface 13 and 16, as by means of a brush, a spatula, byspraying, or by hand.

Said resin is most desirably an unsaturated polyester selected from'anyof those commercially available products sold under the trade nameParaplex, Laminac, Selectron, Marco, or the like, as well known to thoseskilled in the plastics art.

The purpose of the first layer of cloth 15 is to provide a weather-proofcoating for the reflective side of the reflector, to protect the metalcoating 17 from oil and corrosion, and to provide a body which isconveniently and easily removed from the mold upon completion of theproduct. For such purpose a fiberglass woven mat of approximately tenthousandths of an inch thickness has been found advantageous.

The initial layer of fiberglass 15, together with the resin impregnatedtherein and thereunder, is next cured as under an ultraviolet lamp,which assists the activity of the lightactivated catalysts, if any,which may be incorporated in the resin, within the skill of thosefamiliar with the art, or such curing may be accomplished in anyconventional fashion, as in an oven for approximately an hour, atapproximately 200 degrees Fahrenheit, or other satisfactory temperature.

After the curing of the fiberglass impregnated resin, the exposed uppersurface of the fiberglass mat 15 is subjected to a sandblasting. Suchsandblasting provides a satisfactory surface for the next operation,which is the application, by means of a metal spray, of a layer of metalof substantially uniform thickness over the entire convex surface of themold and cloth layer 15. A layer of aluminum has been found satisfactoryfor the instant purposes if deposited, for example, in a layer ofapproximately ten thousandths of an inch. Other metals may also beemployed, but if so, they should be selected from those which do notrequire such a high temperature for spraying that the resultant heat ofthe sprayed metal will deleteriously or otherwise affect the resin.

To the upper surface of the metal layer 17 there is applied, with orwithout sandblasting, and preferably also but not necessarily, a coatingas by brush, spatula, by spray or by hand, a layer of said resin, overwhich is similarly laid a fiberglass mat 18, preferably of wovenfiberglass. Said layer 18 has been found satisfactory if comprised ofthree layers of such fiberglass fabric of about thirty thousandths of aninch combined thickness. Also satisfactory is a single fiberglass mat ofWoved figerglass fibers of approximately the sarne total thickness.

Over the last named layer 18 is placed a preferably honeycomb core 19.If it be assumed in the instant example that the diameter of thereflector is to be approximately eight to nine feet, a honeycomb core ofapproximately one and threeqnarter inches in thickness has been foundsuitable. The honeycomb itself comprises woven fiberglass impregnatedwith a polyester resin. The individual cells of the core. by way ofexample, may be hexagonal and extend continuously from the inside to theoutside walls of the core. The walls of the core defining thehoneycomb-like chambers are approximately three thousandths of an inchin thickness and are impregnated with said resin to give them a totalthickness, including said resin,

of about five thousandths of an inch.

After the honeycomb has been positioned over the layer 18, preferablythough not necessarily as a continuous unit, a further backing 20 ofwoven fiberglass fabric impregnated with said resin is laid over thecore 19. Such v a backing has been found satisfactory in the form ofapproximately four layers or plies of woven fiberglass, each of whichply has an approximate thickness of ten thousandths of an inch, thewhole backing 20 thereby having an over-all thickness of approximatelyforty thousandths of an inch.

After the positioning of the layers 18, 19 and 20, over the sprayedmetal layer 17, the upper surface of the mold is covered with a flexiblebag 21 of any material such as polyvinyl acetate. or the like,non-compatible material which does not, like rubber, inhibit curing ofthe resin, and which does not. after such curing adhere to the resin.

Said bag 21 is laid also over the upper surface of the correspondinglayers 16 through 20 disposed upon the flange 14, and if desired, alsoover the outermost edge 22 of the flange 12, but prior to thepositioning of the bag 21, a porous or otherwise open tube 23 is coiledabout the mold. Such tube 23 may take the form merely of a coil springor other coil of material comprising a helix. Moreover. at the outeredges of the flange, as at 24 and 25, a layer of sealing compound, suchas zinc chromate, is employed, having the desired quality that it doesnot flow under the contemplated heat to which the mold is to besubjected.

The bag 2] is pressed into position over the zinc chromate seal. whichseal, however, is not deposited inwardly, as at 26, between the tube 23and the convex portion 13 of the mold.

A vacuum is next interconnected with the tube 23. The vacuum heretoforeemployed has been that required to subject the dome 27, or convexity, toa pressure of approximately nine pounds per square inch. While thevacuum is applied, any air under the dome portion 27 is rubbed out as bybrushing, rubbing, rolling or the like, with pressure against the top ofthe dome, beginning from the upper more central parts to the outer edgestoward the tube 23. Also, preferably simultaneously with said rubbing,and at least while the vacuum remains applied, the part is cured underheat, for example, 100 degrees Fahrenheit,

for approximately one hour, either in an oven, under ul tra-violetlight, or otherwise.

Thereupon the cured part is removed from the mold by physical strippingafter the bag 21 has been removed. The edges designated collectively at28 overlying the flange 14 are trimmed off, and the raw edges 29 thusexposed are capped as by two layers 3t)31 of woven fiberglass mat, eachhaving a thickness of approximately ten thousandths of an inch,impregnated with said resin, as under a heat lamp, to approximately 200degrees Fahrenheit, for approximately an hour.

A reflector made as above described, by way of example, may have adiameter of approximately nine feet. The over-all thickness, which maybe substantially the same from edge to edge, is approximately 1.84inches, not counting the added thickness of the cap 3031. Theversatility of the instant construction is such that a reflector may bemade in any diameter, as for example, from two inches to fifty or evenlOO feet in diameter. The curve of the reflector surface is preferablyparabolic. The weight of a corresponding steel frame of effectivenine-foot diameter is approximately 350 pounds; whereas, thecorresponding weight of an effective nine-foot reflector made inaccordance with this invention is only ninety pounds. Even the cost ofthe instant reflector is substantially less than that of its steelcounterpart.

A reflector made in accordance with the teaching of this invention has,under test, withstood a 120 mile-per-hour gale directed axially at same,or while rotating the same, with less than one-eighth of an inchdeflection While mounted on a standard support.

it has excellent weather-resistant qualities and may be used eitherexposed or under the protection of a radome or like shelter, inaircraft, aboard ship, or as part of a stationary or mobile land unit.

Although l have herein shown and described my invention in what I haveconceived to be the most practical and preferred embodiment, it isrecognized that departures may be made therefrom within the scope of myinvention, which is not to be limited to the details disclosed hereinbut is to be accorded the full scope of the claims so as to embrace anyand all equivalent structures and methods.

I claim:

1. A radar reflector comprising a generally dish-shaped structure, whichstructure, proceeding from the inner side thereof outwardly through thewall thereof, comprises a continuous layer of a fiber reinforcedresinous laminate, a thin layer of metal, and a resinous cellularmaterial substantially thicker than the metal, said fiber reinforcedlayer and said cellular material being securely adhered to said layer ofmetal by means of a resin interposed therebetween.

2. A reflector as defined in claim 2, and wherein said cellular materialcomprises a substantially uniform layer several times thicker than saidmetal layer, and fiber reinforced resinous layers on the inner and outersides of said layer of cellular material, and respectively firmlyadhered to the outside of the metal layer and the outside of the layerof cellular material, all of said layers being securely and intimatelyadhered to one another by means of a resin impregnating said fiberreinforced layers and the inner and outer surfaces of all of saidlayers.

3. A reflector as defined in claim 2, wherein said cellular materiallayer comprises a honeycomb structure having hollow cells whose axes aredirected radially of the reflector.

4. A reflector as defined in claim 3, wherein said metal layer isapproximately ten-thousandths of an inch thick.

5. A reflector as defined in claim 4, wherein said metal layer isaluminum.

6. A reflector as defined in claim 5, wherein said resin is anunsaturated polyester.

7. A radar reflector comprising a generally dish-shaped structure, whichstructure, proceeding from the inner side thereof outwardly through thewall thereof, comprises References Cited in the file of this patentUNITED STATES PATENTS 2,265,796 Boersch Dec. 9, 1941 10 6 Daly May 13,1947 Mautner May 31, 1949 Hudspeth et a1 Nov. 27, 1951 Cooper Oct. 14,1952 McAuley et al May 19, 1953 Hahn June 29, 1954 FOREIGN PATENTS GreatBritain May 21, 1929 Great Britain Nov. 7, 1946

1. A RADAR REFLECTOR COMPRISING A GENERALLY DISH-SHAPED STRUCTURE, WHICHSTRUCTURE, PROCEEDING FROM THE INNER SIDE THEREOF OUTWARDLY THROUGH THEWALL THEREOF, COMPRISES A CONTINUOUS LAYER OF A FIBER REINFORCEDRESINOUS LAMINATE, A THIN LAYER OF METAL, AND RESINOUS CELLULAR MATERIALSUBSTANTIALLY THICKER THAN THE METAL, SAID FIBER REINFORCED LAYER ANDSAID CELLULAR MATERIAL BEING SECURELY ADHERED TO SAID LAYER OF METAL BYMEANS OF A RESIN INTERPOSED THEREBETWEEN.